The present disclosure relates generally to the field of navigation. More specifically, the present disclosure relates to a system for and method of navigation using weather radar.
Required navigation performance (RNP) specifications for Federal Aviation Administration (FAA) approach procedures require accurate navigation performance. Conventional aircraft often rely on Global Positioning Systems (GPS) and Inertial Navigation System (INS) to estimate the aircraft's position (latitude, longitude and altitude) and other flight parameters (e.g., track angle, ground speed, pitch, roll, heading, angular rates and accelerations). In addition, the RNP specifications require navigation accuracy to be maintained after a failure or loss of any signal on the aircraft or the loss of the GPS signal in space. This generally requires redundancy for navigation sources in case of a GPS or other sensor/system outage or failure. This includes undetected hazardously misleading failures in any of the sensor systems or in any satellite. In general, conventional systems rely upon an INS and accompanying sensors to provide redundant parameters for navigation. The INS itself is expensive. Sensors for an INS generally must be avionic grade and can also add significant cost. An INS system and associated sensors also increase the weight of the aircraft. Additionally, radio navigation sources such as Global Navigation Satellite Systems (GNSS) including GPS, Galileo, GLONASS, and other proposed systems such as COMPASS, are absolute navigation systems. The accuracy of absolute radio navigation sources depends on factors such as satellite geometry, signal propagation effects, noise and transmission errors. In general these systems provide an absolute position reference with a bounded and predictable level of error and do not drift over time. Relative navigation systems such as INS and Doppler Aidied Navigation Systems (DANS) calculate position by integrating acceleration and/or velocity and adding the estimated change in position to an initial position to estimate current position. In these relative navigation systems, sensor errors will cause drift in position error gradually increasing in error over time. Therefore relative positioning systems must be corrected periodically using navigation correction information derived from an absolute position reference like GPS, to ensure that the system position estimate remains inside the specified RNP accuracy limits.
Many current avionics installations utilize a system consisting of two subsystems (left pilot side and right copilot side), each subsystem including an Inertial Reference System (IRS), a GPS receiver and a Flight Management System (FMS). The FMS on each side utilizes the on-side GPS data, when marked valid and accurate, to calculate the position offset of the IRS. If GPS data is lost, the FMS will continue to output a position estimate based on the current output of the IRS and the position offset calculated at the time of last good GPS data. This approach has two significant limitations. The first limitation is that if the GPS receiver has a failure that causes it to produces an erroneous output, the FMS will output an erroneous position and calculate an erroneous position offset estimate for the IRS. Although cross monitoring techniques may be used to detect when the left and right FMS position estimates diverge, the system does not provide an automatic method for determining which FMS solution is erroneous and which is accurate. The second limitation is that the FMS calculates only a position offset in the IRS solution, no sensor error parameters are updated in the IRS so the rate of drift is not reduced. After a long flight the IRS may have a significant rate of position drift. This reduces the time that the system can accurately “coast” after loss of GPS.
Both of the above limitations can impact safety of RNP navigation to tight limits such as RNP 0.1 nautical miles. When operating an aircraft within tight RNP limits there is little time for a pilot to determine which GPS system has failed and which system is accurate after a position miscompare is identified, and high rates of IRS drift greatly limit the time the aircraft can maintain the RNP limits after loss of GPS.
Therefore, there is a need for a system and method that can quickly and automatically identify and isolate GPS receiver failures and which provides high-integrity feedback to the inertial system to allow sensor errors to be calibrated to enhance coasting performance after loss of GPS.